1. Field of the Invention
The present invention relates to receiver for a telecommunications system, and to a method of recovering a pilot signal from a received signal.
2. Description of Related Art
In the High Speed Downlink Packet Access (HSDPA) protocol used in third generation (3G) cellular communications systems a common pilot channel (CPICH) and a data channel, known as the High Speed Downlink Shared Channel (HS-DSCH) are transmitted by the network to a receiver such as a mobile telephone. The CPICH contains a known sequence of symbols which can be used for a number of purposes including channel impulse response estimation and link quality measurement, whilst the HS-DSCH contains data such as encoded voice signals.
To improve the reliability of signals transmitted by the network, diversity schemes are used. The CPICH is commonly transmitted using an open loop space-time-block-coding-based transmit-antenna diversity (STTD) scheme, in which the same information is transmitted using two antennae, with the signal transmitted by one antenna being an altered version of the signal transmitted by the other. The HS-DSCH is also transmitted using a diversity scheme which may be STTD, but is often a closed loop transmit diversity scheme in which two antennae are used to transmit the same information, with the signal transmitted by one of the antennae being a phase-shifted version of the signal transmitted by the other antenna.
The HSDPA protocol can give rise to complicated receiver designs. An equalizer is required in the receiver to negate the effects of the propagation channel through which the HS-DSCH is received. If the HS-DSCH is transmitted using a closed loop transmit diversity scheme, the HS-DSCH can be treated as a single signal being transmitted through a single composite propagation channel, and thus only a single equalizer is needed to equalize (i.e. negate the effects of the propagation channel on) the HS-DSCH to permit accurate recovery of the HS-DSCH.
The CPICH is always transmitted using a STTD scheme when a closed loop transmit diversity scheme is used to transmit the HS-DSCH. This effectively involves generating two slightly different CPICH streams (a process known as STTD encoding) and transmitting the two CPICH streams over two separate propagation channels. An equalizer configured to equalize a signal transmitted using closed loop transmit diversity cannot properly equalize an STTD encoded signal. Thus, in order for a receiver accurately to decode the two separate versions of the CPICH, separate equalizers are required to equalize the signals received through the two separate propagation channels. Thus, two equalizers are required to receive and decode the CPICH accurately.
An HSDPA-compatible receiver must therefore include three equalizers, to ensure that it can accurately recover both the CPICH and the HS-DSCH regardless of the transmit diversity scheme used to transmit the HS-DSCH. Such equalizers are undesirably complex and power-inefficient, as they require a large number of components.